Human Vision
Normal human vision provides a perception of space in the visual field of view that is in color and three dimensions (3D). A better realization of the optical requirements for a photographic system to present an acceptable 3D stereoscopic image or stereo-model to the viewer is given by an understanding of stereopsis, or visual perception of space.
The stimulus conditions for space perception are termed cues, and are in two groups. The monocular group allows stereopsis with one eye and includes relative sizes of subjects, their interposition, linear and aerial perspective, distribution of light and shade, movement parallax of subject and background and visual accommodation. The binocular group uses the two coordinated activities of both eyes: firstly, visual convergence, where the optical axes converge muscularly from parallel for distant vision to a convergence angle of 23.degree. for a near point of 150 mm; and secondly, stereoscopic vision, where, due to the two different visual viewpoints, the imaging geometry gives two disparate retinal images for the left and right eyes. The disparities are due to parallax, the relative displacement of corresponding or homologous image points of a subject point away from the optical axis due to its position in the binocular field of view.
Retinal images are encoded for transmission as frequency modulated voltage impulses along the optic nerve, with signal processing taking place at the intermediate lateral geniculate bodies and then the visual cortex of the brain. The resultant visual perception is unique to the observer. For a further discussion of human 3D perception, see, e.g., Sidney F. Ray, "Applied Photographic Optics Imaging Systems For Photography, Film and Video," Focal Press, pp. 469-484, (1988), which is incorporated herein by reference.